The best resource for this kind of thing is Tom's Hardware forums. Someone has almost certainly had your exact problem and posted a solution. The community there is really knowledgeable and helpful.

I'd post this on the Tom's Hardware or AnandTech forums with your full system specs — CPU, RAM, motherboard, the works. People can't really help without knowing what you've got. The more detail you give, the better the answers.

Honestly at some point a clean reinstall of Windows fixes more than any amount of tinkering. XP gets crufty after a couple years. Back up your files, find your product key, and do a fresh install. It'll feel like a brand new machine.

I'd recommend trying the free solution first before spending money. Nine times out of ten, the free option works just as well as the paid one. Only upgrade to paid software if the free version doesn't meet your needs.

Check how much free space is on your hard drive. Windows needs at least 15% free to run well — it uses that space for virtual memory and the swap file. If your drive is 95% full, that alone will make everything crawl.

Definitely back up your important files before trying any fixes. I learned this the hard way when I lost 3 years of photos trying to fix a simple problem. Get an external hard drive or burn DVDs of your important stuff first.

Newegg is your friend for parts. Way cheaper than Best Buy or CompUSA and the customer reviews actually help you avoid junk. Just watch out for shipping on heavy stuff like CRT monitors. Read the reviews before you buy ANYTHING.